1. Field of the Invention
This invention relates to distance measuring devices, and, more particularly, to a distance measuring device having light projecting means provided therein to project light to a target, and in which the concurrent reflected light from the target is received by photoelectric means provided with a plurality of photosensitive elements operative independently of each other and arranged in predetermined spaced relation to the above described light projecting means so as to correspond to respective different target distances. In this manner the output of the light receiving means is processed to recognize the distance to the above described target.
2. Background of the Prior Art
Distance measuring devices of the so-called "active" type having light projecting means provided on the device side to project light to a target with the concurrent reflected light from the target being utilized to detect the distance to the target have been very often proposed in connection with the automatic in-focus detecting and automatic focusing devices for use in cameras.
For example, among others, the applicants of the present invention have proposed a distance measuring device of the active type as applied to an automatic focusing device for use in a camera in Japanese Patent Laid-Open Specification No. Sho 49-49625. According to this proposal, the photoelectric light receiving means for receiving the reflected light from the above described target is provided with a plurality of photosensitive elements independent of each other, and these elements are arranged in respective positions spaced from the light projecting means by predetermined base line distances so as to correspond to different target distances from each other. From the state of the output of this photoelectric light receiving means, detection is made as to which photosensitive element receives the reflected light from the target to measure the distance to said target. For distance detection, the device of such construction operates in such a manner that the light projecting means first projects light to the target while simultaneously permitting the outputs of the individual photosensitive elements to be stored on respective condensers. Then, after that, the light projection from the light projecting means is stopped, with the resultant outputs of the photosensitive elements being compared with the previously memorized values on the condensers by means of respective difference amplifiers.
By sensing a concurrent output from one of the difference amplifiers, which of the above described plurality of photosensitive elements receives the reflected light from the target is detected so as to provide measure of the distance to the target. By this method, it is made possible to effectively remove the influence of light other than the projection light (that is, the light serving as the external disturbing light in this kind of device to give rise to a loss in the accuracy of distance measurement), and therefore to achieve an increase in the accuracy of distance measurement.
The device according to this proposal requires, much improvement. For example, in this device, the light receiving element is formed with a light responsive resistor such as a CdS, and from the circuit point of view, the individual light receiving elements are connected to respective resistors with the junction points therebetween, that is, the potential dividing points being connected to respective storing condensers. The voltages on the individual condensers are applied to respective individual difference amplifiers at one of the inputs thereof, the opposite inputs of which are supplied with the voltages from the potential dividing points. It is noted here that since each condenser memorizes the voltage dependent upon the resistance value of each photosensitive element, in order to obtain a voltage of level high enough to assure the detection, the intensity of light projected from the light projecting means must be increased. In application to instruments of small size and particularly to cameras employing an electrical power source of limited capacity there are yet much more problems left to be overcome. Again, along with this, when the environment is bright, the potentials at the above described potential dividing points become high even in the state where the light projection from the light projecting means is cut off. As a result, the level of noise component is increased, and, therefore, the output level of the difference amplifier is considerably reduced. Thus, there is a high possibility of encountering situations where the distance measurement is impossible.
Another problem is that, as the environmental illumination varies to a large extent from a time point at which light is projected from the light projecting means to a time point at which the light projection is stopped, because this variation can not be compensated for, an error operation will be resulted. Besides this, since the output terminals of the difference amplifiers are scanned successively by a slide tap to detect the effective output representative of the actual distance to the target, if the reflected light from the target is incident upon two adjacentones of the photosensitive elements at a time, and therefore the corresponding two difference amplifiers produce effective outputs, the scanning operation by the slide tap results in detecting the output terminal of the first difference amplifier counting in a direction in which the slide tap is moved to scan regardless of which photosensitive element of the two receives more light. At this time, when the photosensitive element cooperative with the second difference amplifier receives more reflected light, no true distance measurement can be effected.